A random access channel (RACH) is a channel for acquiring initial uplink synchronization. If a terminal (i.e., UE) is firstly powered on, or the terminal (UE) is switched to an active status after it has been in an idle mode for a long period of time, uplink synchronization should be re-established. The RACH is generally adapted to re-establish the uplink synchronization, and need not establish time synchronization or frequency synchronization.
The RACH basically supports a plurality of users (hereinafter referred to as a multi-user). Each user equipment (UE) transmits a specific preamble sequence when accessing the RACH. If a base station (hereinafter referred to as a Node-B) recognizes the preamble sequence and transmits the recognized preamble sequence, the user equipment (UE) updates its own time synchronization information using the aforementioned preamble sequence information. In this case, if the Node-B transmits frequency synchronization information along with the time synchronization information, this frequency synchronization information can also be used to correct the user equipment (UE).
A Constant Amplitude Zero Auto-Correlation (CAZAC) sequence is a representative one of various sequences which have been intensively discussed in the 3GPP LTE.
Channels generally extract a variety of identifiers (IDs) or information using the CAZAC sequence, for example, synchronization channels (e.g., a primary-SCH, a secondary-SCH, and a BCH) for downlink synchronization, other synchronization channels (e.g., a RACH) for uplink synchronization, and pilot channels (e.g., a data pilot, and a channel quality pilot). Also, the above-mentioned CAZAC sequence has been used to perform the scrambling.
The CAZAC sequence is generally classified into the GCL CAZAC sequence and the Zadoff-Chu CAZAC sequence. The GCL CAZAC sequence and the Zadoff-Chu CAZAC sequence have complex conjugates with each other. The GCL CAZAC sequence may be acquired by a complex conjugate of the Zadoff-Chu CAZAC sequence. The Zadoff-Chu CAZAC sequence can be represented by the following equations 1 and 2:
                              c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                              exp            ⁡                          (                                                jπ                  ⁢                                                                          ⁢                                      Mk                    ⁡                                          (                                              k                        +                        1                                            )                                                                      N                            )                                ⁢                      (                          for              ⁢                                                          ⁢              odd              ⁢                                                          ⁢              N                        )                                              (                  Equation          ⁢                                          ⁢          1                )            
                              c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                              exp            ⁡                          (                                                jπ                  ⁢                                                                          ⁢                                      Mk                    2                                                  N                            )                                ⁢                      (                          for              ⁢                                                          ⁢              even              ⁢                                                          ⁢              N                        )                                              (                  Equation          ⁢                                          ⁢          2                )            
In Equations 1 and 2, “k” is indicative of a sequence index, “N” is indicative of the length of the CAZAC sequence to be generated, and “M” is indicative of a sequence ID.
If the Zadoff-Chu CAZAC sequence of Equations 1 and 2 and the GCL CAZAC sequence which is complex-conjugated with the Zadoff-Chu CAZAC sequence are denoted by c(k; N,M), the following equations 3, 4, and 5 are acquired.|c(k;N;M)|=1 (for all k,N,M)  [Equation 3]
                                          R                          M              :              N                                ⁡                      (            d            )                          =                  {                                                                      1                  ,                                                                              (                                                            for                      ⁢                                                                                          ⁢                      d                                        =                    0                                    )                                                                                                      0                  ,                                                                              (                                                            for                      ⁢                                                                                          ⁢                      d                                        ≠                    0                                    )                                                                                        (                  Equation          ⁢                                          ⁢          4                )            RM1,M2;N(d)=p (for all M1,M2 and N)  [Equation 5]
In Equation 3, the size or amplitude of the CAZAC sequence is always indicative of “1”. In Equation 4, the auto-correlation function of the CAZAC sequence is denoted by a delta function, and the auto-correlation is based on a circular correlation. In Equation 5, a cross-correlation is always indicative of a constant or invariable number.
The CAZAC sequence is a representative sequence which has been intensively discussed in the 3GPP LTE. The CAZAC sequence can be generally used by the following methods. A first method changes a root index of the sequence to another, and then uses the resultant sequence. A second method performs the circular shift (CS) on the sequence corresponding to a single root index, and then uses the resultant sequences.
There are two methods for applying the circular shift (CS) to the CAZAC sequence, i.e., a first method for performing the circular shift (CS) on the sequence, and a second method for multiplying an exponential function of another area by a time-area sequence or a frequency-area sequence, and then performing the circular shift (CS) on the multiplied result.
The circular shift (CS) method of the sequence can be represented by the following equation 6:c(k;d,M,N)=c(mod(k−d,N);M,N)  [Equation 6]
In Equation 6, “d” is indicative of an amount of the circular shift (CS), and “mod” is indicative of a modulo-operator.
The method for multiplying the sequence by an exponential function, and applying the circular shift (CS) to the resultant sequence can be represented by the following equation 7:
                              c          ⁡                      (                                          k                ;                d                            ,              M              ,              N                        )                          =                              f            ⁡                          (                                                                    mod                    ⁡                                          (                                                                        k                          -                          d                                                ,                        N                                            )                                                        ;                  M                                ,                N                            )                                =                                    exp              ⁡                              (                                                      j2π                    ⁢                                                                                  ⁢                    dk                                    N                                )                                      ⁢                          FFT              ⁡                              (                                  c                  ⁡                                      (                                                                  k                        ;                        d                                            ,                      M                      ,                      N                                        )                                                  )                                                                        (                  Equation          ⁢                                          ⁢          7                )            
The CAZAC sequence has a little cross-correlation value under different root indexes, but this cross-correlation does not affect the design of the sequence usage.
However, if the circular shift (CS) is applied to the CAZAC sequences, a cross-correlation value among the resultant CAZAC sequences is zero, so that these CAZAC sequences are used when the high rejection ratio is needed. Specifically, the above CS-processed CAZAC sequences share the same time-frequency resources within the same cell, so that they are used to discriminate among different signals and different UEs during the transmission of data/control signals.
For the convenience of description and better understanding of the present invention, the above-mentioned Zadoff-Chu CAZAC sequence is called a ZC sequence. If a specific sequence of the ZC sequence is discriminated by only the root index irrespective of the circular shift (CS), this specific sequence is called a root index sequence or a root sequence. If the circular shift (CS) is applied to a specific sequence of the specific root index sequence such that the specific sequence is discriminated, this specific sequence is called a circular shift (CS) sequence or a zero correlation zone (ZCZ) sequence. In this case, the ZCZ is indicative of a specific interval which can discriminate among sequences to which different circular shifts (CSs) are applied although the circular shift (CS) is applied to a specific root index sequence.
In the meantime, a mobile communication system may support a variety of circular shift sizes of a specific sequence. For example, the above-mentioned random access channel (RACH) may have different circular shift sizes for the ZC-ZCZ sequences according to the sizes of individual cells.
For example, for the convenience of description, it is assumed that the length of the ZC sequence is N samples, and each cell must provide M number of random access signals (hereinafter referred to as M number of random access opportunities) capable of being simultaneously supported. Therefore, a cell of using only a single small-sized root index must provide M/1 number of circular shift (CS) sequences (i.e. M/1 CS sequences) for each root index sequence. Also, the cell of using the K number of root indexes must provide ┌M/K┐ number of circular shift (CS) sequences for each root index. In this way, there is a need to provide a variety of circular shift (CS) sequences according to the number of root indexes used for either each cell or each cell size.
In addition, in some cases, there may be needed to use different sequences according to individual cells or individual areas. Therefore, in order to allow a cell of a restricted circular shift (CS) cell or a specific-region UE to recognize sequence-type information for a random access, the conventional RACH requires the additional signaling.